The incidence of drug-resistant parasitic protozoal diseases has grown significantly in recent years resulting in an increased number of deaths in both developing countries and the Western world. Strategies being developed to address the problem include the development of new drugs, the adoption of strict treatment regimens and a comprehensive public education programme.
A possible means for delivering highly active pharmacological agents to their site of action with minimal unwanted side effects in other cells and tissues is the use of prodrugs. Prodrugs have been known for many years and are used in several medical indications. They can be defined as chemical entities which are modified by metabolic or non-metabolic systems resulting in the formation or liberation of a species with the desired pharmacological activity. In many cases, prodrug forms are used to modify drug pharmacokinetics by the alternation of a physicochemical property of the drug (such as lipophilicity). In these cases, modification of the prodrug is generally non-specific, occurring by non-enzymatic degradative processes or by the metabolic action of ubiquitous enzymes. Prodrug forms may also be used, however, to target the active pharmacological agent to specific sites only, generally by exploiting the differential distribution of enzymes capable of catalysing the prodrug modification reaction.
In the field of cancer chemotherapy activation of prodrugs by target-specific enzymes has long been a goal and many potential prodrugs have been synthesised and tested in the hope that a tumour-specific enzyme would be capable of converting them specifically into a potent anti-tumour agent.
Many researchers are still active in this area utilising prodrugs designed to be activated by a variety of enzymes including β-glucuronidase (Woessner et al (2000) Anticancer Research 20, 2289-2296), DT diaphorase (Loadman et al (2000) Biochemical Pharmacology 59, 831-837) and thymidylate synthase (Collins et al (1999) Clinical Cancer Research 5, 1976-1981).
As an alternative to this prodrug monotherapy strategy some researchers have attempted to deliver the desired enzyme to the tumour, prior to administering a prodrug. Such an approach, often termed antibody-directed enzyme prodrug therapy (ADEPT), has been disclosed in WO88/07378. In this example the enzyme is linked to a monoclonal antibody which is capable of binding to a tumour-associated antigen. In this way the enzyme is delivered to the tumour site where it can act on an appropriate prodrug. A similar strategy has been disclosed, for example, in WO 96/03151, in which the gene encoding an enzyme is delivered to the tumour and, once present, expresses the desired enzyme. This strategy is often termed gene-directed enzyme prodrug therapy (GDEPT).
It is an important tenet of many of these targeting strategies that the enzyme delivered to the tumour site should not be endogenous to the host (Aghi et al (2000) J Gene Med 2, 148-164). The presence of endogenous enzymes would lead to non-specific activation of the prodrugs and toxicity to normal tissues. Accordingly, the majority of these enzyme-prodrug therapy studies have been carried out using bacterial enzymes such as carboxypeptidase G2, β-lactamase and nitroreductase.
Whilst much of the work to date in the field of site-specific prodrug activation has concentrated on anticancer therapy, there have been recent efforts to adapt this technology for use in antibacterial applications. For example, WO 99/32113 describes the use of prodrugs consisting of a cytotoxic moiety and a β-lactam moiety. In Gram-negative bacteria containing β-lactamase enzymes within the periplasmic space, the prodrug is cleaved to release the cytotoxic moiety which then goes on to disrupt vital cell functions. These prodrugs are limited in their breadth of applicability, however, in that not all bacteria express β-lactamases and in that Gram-positive bacteria tend to excrete β-lactamases such that much of the beneficial toxin-locating effect of the prodrug will be lost.
In Smyth et al ((1998) J. Org. Chem. 63, 7600-7618), the S-aminosulfenimino-penicillins are introduced. These compounds are both β-lactamase inhibitors and potential prodrug templates for the delivery of a variety of agents into β-lactamase-positive bacteria. As with the β-lactam-derivatives discussed above, however, these compounds are likely to be of limited utility in Gram positive bacteria and, due to their slow permeation through outer membrane porin structures, may need to be present at undesirably high extracellular concentrations in order to achieve significant effects in Gram-negative bacteria.
A departure from the potential shortcomings of β-lactamase-dependent prodrugs has been presented by Wei and Pei ((2000) Bioorg. Med. Chem. Lett. 10, 1073-1076). These workers conceptually demonstrated the use of 5′-dipeptidyl derivatives of cytotoxic antibacterial agents as prodrugs activatable by bacterial peptide deformylase. Preliminary results using these prodrugs suggested rather weak antibacterial activity, however, and it was hypothesised that this may have been due to poor uptake of the compounds into cells.
Human parasitic diseases are endemic in many parts of the world. For example, leishmaniasis (a parasitic disease caused by an obligate intracellular protozoan transmitted by the bite of some species of sand flies) is found in approximately 90 tropical and subtropical countries around the world and in southern Europe. More than 90% of the world's cases of, cutaneous leishmaniasis are in Afghanistan, Algeria, Brazil, Iran, Iraq, Peru, Saudi Arabia, and Syria. However, approximately 75% of the cases that are evaluated in the United States were acquired in Latin America, where leishmaniasis occurs from northern Mexico (occasionally in rural southern Texas) to northern Argentina. More than 90% of the world's cases of visceral leishmaniasis occur in Bangladesh, Brazil, India, Nepal, and Sudan. Similarly, the geographical distribution of Chagas' Disease (caused by a flagellate protozoan parasite, Trypanosoma cruzi, transmitted to humans by triatomine insects) extends from Mexico to the south of Argentina. The disease affects 16-18 million people and some 100 million, i.e. about 25% of the population of Latin America, is at risk of acquiring Chagas' disease. Even people staying for a short time in parasite-endemic areas can become infected and parasitic diseases are becoming a problem in the developed world as a result from the increase in global travel.
Resistance to the presently used drugs has been reported. Problems of resistance require the use of more toxic drugs and as the drugs are becoming less and less effective new drug discovery is needed.